Langerhans cells are a type of dendritic cell uniquely positioned to play a vital role in immunity at the barrier surfaces. They form a network at the base of stratified epithelium in the skin. They have potent antigen presenting function in vitro, and promote robust immune responses in vivo, as we showed in the previous funding period. However, they are also thought to play a role in tolerance and immunological suppression, particularly after low dose UVB irradiation. There is also growing interest in the role of Langerhans cells in epicutaneous immunization, a strategy that shows surprising efficacy against viral and bacterial infection, and tumors. Nonetheless/many basic properties of Langerhans cells are uncharacterized at present: including whether they can cross-present MHCClass I antigens, if they can induce peripheral tolerance, and how their immunological properties are changed after insult or injury to the skin. Fortunately, the field is rapidly developing incisive tools to study this population. In the current application we propose to utilize someof these new tools and develop others, that together will allow us to answer specific questions about the role of Langerhans cells (LC) in peripheral tolerance to epidermal antigens. We propose to create and study TCR transgenics with high and low affinity for a naturally occurring epidermal antigen: tryosinase related protein 2 (trp2). We will define the ARC responsible for processing and presentation of trp2180-188, using bone marrow chimeras and inducibly LC deficient mice. We will also study the role of Langerhans cells in tolerance and immunity to exogenousantigens introduced through the skin (epicutaneous immunization or ECI) by developing an ovalbumin-based model system. Using this we will be able to test specific hypotheses about how skin immune response modifiers, Imiquimod, CPG DNA, and UVB irradiation, influence immunity and tolerance.